1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a printer that produce images by visualizing electrostatic images formed on an image bearing member.
2. Description of the Related Art
Recently, as a POD (print on demand) market expands, an electrophotographic image forming apparatus makes an attempt to enter the POD market. An apparatus of higher productivity (a larger number of output prints per unit time) is expected to be introduced.
On the other hand, however, since reduction of power consumption is also required in order to cope with environmental issues, it is not allowed to increase the power consumption largely for increasing the printing speed. Therefore, it is desired to achieve an increase in printing speed and reduction in power consumption at the same time. It is needless to say that a high quality image formation is expected also in terms of image quality.
Under such circumstances, there are large differences between the printing and the electrophotography that uses a toner to form images. One of the differences is a “toner relief” which occurs during image forming. Unlike the printing which uses an ink as a liquid, in the electrophotography in which a toner of powder in nature is fused and fixed onto a transfer material such as a paper by a fixing device with pressure and heat, even the fixed toner has a volume to a certain extent. Consequently, when a high-density portion of a larger toner amount is adjacent to a low-density portion of a smaller toner amount, in a large case, a toner relief of 10 μm or more occurs resulting in an uneven touch on images. The uneven touch may give an undesirable feeling to users who are accustomed to a substantially plane print surface. Therefore, it is desired to be capable of forming images with less toner relief.
In the POD market, particularly, there are requests to use thin papers. For example, it is conceivable that there may be a case that full color images are formed on a thin paper of 40 to 50 g/m2 or less without changing the throughput. However, when images are formed on such a thin paper using a conventional toner amount (toner bearing amount), elasticity of the paper tends to get defeated by a force, which is generated due to a phase change of the toner during fixing process, resulting in a curl generated on the paper. The “phase change of the toner” is a phenomenon in which a powder toner is fused once, and then solidified again to be fixed on a transfer material like a paper. Also the “curl” is a phenomenon such that a transfer material such as a paper fixed with the toner forms a curvature; and generally refers to a phenomenon such that the side, on which the toner exists, of the transfer material such as a paper fixed with the toner forms a curvature into a concave or downwardly rounded surface.
Further, it is strongly requested to reduce the running cost per sheet of color images.
The inventors examined and found that, in order to respond such requests, it is one of the extremely effective techniques to largely reduce the toner amount (toner bearing amount) needed for image forming.
For example, the fixing temperature may be reduced by several dozen degrees by reducing the toner bearing amount to a half. Further, by utilizing the power equivalent to the reduction effect of the fixing temperature, the printing speed can be increased with the same power consumption as that of the conventional art. By reducing the total amount of the toner necessary for forming images to a half, a large effect to reduce the toner relief and the curl is obtained. Furthermore, by reducing the amount of the toner used per an output image sheet, the running cost can be also largely reduced.
Thus, reducing the toner bearing amount is extremely effective to increase the productivity and the applicability to thin papers and to achieve an image quality with a smaller toner relief closer to the image quality of the ordinary printing, by use of the electrophotographic method.
Conventionally, a technique to reduce the toner bearing amount by increasing tinting strength of the toner has been proposed (Japanese Patent Application Laid-Open No. 2005-195674).
However, the examination by the inventor et al. revealed that, for example, after enhancing the tinting strength of the toner by increasing the amount of coloring agent contained in the toner, simply reducing the developing contrast by the amount corresponding thereto to reduce the toner bearing amount may cause the following disadvantages to occur.
Referring to FIG. 12A, a relationship between potential and developing bias on an electrophotographic photosensitive member (hereinafter referred to as “photosensitive member”) is illustrated. Developing contrast (Vcont) is a difference between a latent image electrical potential (exposed portion potential) formed on the photosensitive member and a potential Vdc of a DC-component of developing bias in an image forming per one color. The developing bias may be a superimposed voltage of an AC voltage and a DC voltage. Further, a difference between a latent image electrical potential VL formed on the photosensitive member to obtain a maximum toner bearing amount (i.e., maximum density) and the Vdc; i.e., |Vdc−VL| is particularly represented with “Vc” as a maximum value of the developing contrast Vcont (hereinafter also referred to as “maximum developing contrast”). Charge potential (potential in an unexposed portion) of the photosensitive member is represented by “Vd”. Potential difference between charge potential Vd in the photosensitive member and potential Vdc of DC-component of the developing bias; i.e., |Vdc−Vd| is referred to as a fog removal bias (Vb).
(1) Increase of γ
FIG. 2 illustrates a relationship between a transmission density Dt and a developing contrast Vcont in a gradation image formed on a paper as a transfer material through the development, transfer and fixing processes (FIG. 3 is the similar graph). A line “a” in FIG. 2 represents a γ-characteristic (gradation characteristic) obtained using a conventional common toner, which is controlled to obtain a maximum density (Dtmax=1.8) at Vc=150 V (point-p).
In this specification, the density of an image is indicated as a transmission density Dt measured on the fixed image using a transmission densitometer TD904 manufactured by the GretagMacbeth AG. In order to describe a relationship between the toner bearing amount and the density under a condition that the influence of gloss caused from a surface condition of a toner layer on a transfer material was removed, the transmission density ⊃t was used. As for the paper as the transfer material, OK Topcoat (73.3 g/m2) from Oji Paper Co., Ltd was used. In the following descriptions, all the paper used was the above coat paper.
The developing contrast Vcont on the abscissa in FIG. 2 is obtained as a difference between the potential of a digital latent image, which is continuously formed on the photosensitive member with varying gradation, and the potential Vdc of DC-component of the developing bias. In order to facilitate the description, FIG. 14 illustrates the potential of a latent image in the case where the latent image electrical potential of the digital latent image of the gradation image is varied in 17 steps. FIG. 14 also schematically illustrates enlarged images in several gradations. That is, (a) in FIG. 14 represents a maximum density image (solid image). Each of (b), (c) and (d) in FIG. 14 also represents a half-tone image respectively, the density of which is lowered in this order. Further, (e) in FIG. 14 represents a minimum density image (blank copy image); i.e. an area to which no toner should be adhered.
As shown in FIG. 13A, a desired latent image is formed on a photosensitive member 1 with an exposing device 3, and the latent image electrical potential thereof was measured with a surface electrometer Vs disposed at the downstream side than the exposing device 3 in a rotational direction of the photosensitive member 1.
The γ-characteristic indicated with the line “a” in FIG. 2 was obtained when the toner was used in which the tinting strength was adjusted so as to obtain the maximum density (Dtmax=1.8) at approximately 0.56 mg/cm2 of the toner bearing amount on the paper. The value of 0.56 mg/cm2 was the toner bearing amount on the paper. The toner bearing amount here was the value after the toner layer of approximately 0.6 mg/cm2 was formed on the photosensitive member in the developing process and after completing the developing process, and the toner layer was transferred on the paper through the transfer process twice via an intermediate transfer member. In this case, the transfer efficiency after the twice transfer processes was approximately 93%. Also, it is assumed that after the fixing process, there has been no change in the toner bearing amount after the completion of transfer process.
In the case of the γ-characteristic indicated with the line “a” in FIG. 2, when the developing contrast Vcont changes, for example, by 25 V (ΔVcont=25 V), the density Dt changes by 0.15 (Δdt=0.15). That is, when the developing contrast changes by ΔVcont=10 V, the density changes by Δdt=0.06.
Ordinarily, an electrophotographic image forming apparatus has various mechanical or electrical fluctuations. For example, ordinarily, the distance (S-D gap) between the developer carrying member and the photosensitive member varies depending on a mechanical tolerance. Also, ordinarily, the value of the bias applied to the developer carrying member subtly changes. That is, the developing contrast Vcont changes a little due to the mechanical or electrical fluctuation.
Therefore, for example, when an image of fully uniform density is formed, the large change in density with respect to the subtle change of the developing contrast Vcont as described above will cause an uneven image in the same area.
Currently, for the density change of Δdt=0.15 or so with respect to the developing contrast change of ΔVcont=25 V, generally, uniformity in an image area can be ensured.
Contrarily, a line “a′” in FIG. 3 indicates the γ-characteristic in the following case. That is, a toner with a double density of a conventional toner (i.e., tinting strength is twice) was used; the developing contrast was set to a half of a conventional contrast (Vc′=(½)×Vc); and the toner bearing amount was set to approximately a half (maximum toner bearing amount on the paper: 0.28 mg/cm2). In FIG. 3, the identical line “a” shown in FIG. 2 is also illustrated.
The inclination of the γ-characteristic indicated with the line “a′” in FIG. 3 is sharper than that of the line “a”, in order to achieve Dtmax=1.8 by a half toner bearing amount (point-p′) of the case in the γ-characteristic indicated with the line “a”.
In the case of the γ-characteristic indicated with the line “a′”, it is extremely difficult to obtain the gradation. Further, the density change becomes too high as Δdt′=2 Δdt with respect to the above-mentioned developing contrast change of ΔVcont=25 V. As a result, an image including a large unevenness may be resulted in.
(2) Increase of Coarseness
Between the case of the -characteristic indicated with line “a” in FIG. 2 and FIG. 3 and the case of the -characteristic indicated with the line “a” in FIG. 3, coarseness (smoothness of image) in low density portions (halftone portions) each having the same density was compared. As a result, it was found that, in the low density portion (halftone portion) having the -characteristic indicated with the line “a”, the coarseness was largely worsened. The reason of this is understood as described below.
The image in the low density portion (halftone portion) was obtained by developing the latent image electrical potential having a potential indicated with Vh in FIG. 14.
Since Vcont=|Vdc−Vh|≈0, the image has a transmission density at a point in the vicinity of Vcont=0; i.e., approximately Dt=1 in FIG. 2.
The gradation electric potentials in FIG. 14 are latent image electrical potentials of digital latent images obtained while changing the emitting width by PWM (pulse width modulation) in laser exposure. FIG. 14 shows gradation electric potentials obtained based on gradation data of two hundred lines. Therefore, the latent image electrical potential Vh of the actual half-tone image forms non-image areas and image areas alternately, for example, as shown in FIG. 15A. FIG. 15A schematically illustrates an enlarged half-tone image. FIG. 15B schematically illustrates the latent image electrical potential of the half-tone image shown in FIG. 15A.
FIG. 16 schematically illustrates a space electrical potential between the photosensitive member and the developer carrying member. Hereinafter descriptions will be made using the following coordinate system shown in FIG. 16. That is, the main scanning direction (corresponding to the laser scanning direction) is the y-axis; the sub-scanning direction (corresponding to a surface movement direction of the photosensitive member) is the z-axis; and the straight-line direction connecting between the surfaces of the photosensitive member and the developer carrying member is the x-axis. The x-axis, the y-axis and the z-axis are perpendicular to one another.
When the latent image electrical potential Vh on the half-tone image is expressed more precisely, the potential is represented with a repeated potential of Guassian distribution as shown in FIG. 15B. That is, a potential distribution, which has a potential Vha (hereinafter, referred to as “a peak latent image electrical potential in an image area”) as a peak potential at the VL side at substantially central point in the main scanning direction of one image area, is repeated. Average potential Vh is obtained by measuring the latent image electrical potential illustrated in FIG. 15B while maintaining a limited distance using a surface electrometer Vs shown in FIG. 13A.
FIGS. 17A and 17B are diagrams each illustrating a potential (space electrical potential) between the photosensitive member and the developer carrying member, which is plotted from the surface of the photosensitive member to the surface of the developer carrying member. In FIGS. 17A and 17B, the plane “y-z” at x=0 represents the potential distribution shown in FIG. 15B.
In FIGS. 15A, 15B, 16, 17A and 17B, Y1 indicates the identical position in the y-axis direction; i.e., particularly, the substantially central point (a peak of a latent image electrical potential in an image area) in the main scanning direction in one image area of a half-tone image.
FIG. 17A illustrates, as an example, changes of the potential when a developing bias of Vdc=300 V is applied to the latent image electrical potential of Vd=450 V, VL=150 V, Vh=310 V, Vha=170 V (calculated value). In this case, from the following formulae:Vc=|Vdc−VL|=150 V; andVb=|Vdc−Vd|=150 V,Vc is 150 V, and Vb is 150 V.
Actually, a developing bias of a superimposed AC voltage and DC voltage is applied to the developer carrying member. However, the Vdc may be used as an average potential.
FIG. 17B illustrates, as an example, changes of the potential when a developing bias of Vdc=225 V is applied to a latent image electrical potential of Vd=375 V, VL=150 V, Vh=310 V and Vha=170 V (calculated value). In this case, from the following formulae:Vc=|Vdc−VL|=75 V; andVb=|Vdc−Vd|=150 V,Vc is 75 V, and Vb is 150 V.
That is, FIG. 17B illustrates a distribution of the latent image electrical potential when the charge potential Vd and potential Vdc in the DC-component of the developing bias are controlled so that, at the same fog removal bias Vb, Vc′=(½)×Vc with respect to the same image area peak potential Vha as the case of FIG. 17A.
FIG. 18 illustrates an electrical potential distribution, which is extracted at x=40 μm in the space electrical potential shown in FIGS. 17A and 17B; i.e., in a plane (y-z plane) 40 μm away from the photosensitive member toward the developer carrying member. A line “C” in FIG. 18 represents an electrical potential in the y-z plane at x=40 μm in FIG. 17A; while a line “C′” in FIG. 18 represents an electrical potential in the y-z plane at x=40 μm in FIG. 17B. Referring to FIG. 18, it is found that, in the y-direction, the line “C′” has more moderate and wider inclination of the changes of the electrical potential than the line “C”.
FIG. 19 illustrates the changes of the electrical potential, which is extracted from a plane of y=Y1 (x-z plane) in the space electrical potential shown in FIGS. 17A and 17B. A line “b” in FIG. 19 represents the changes of the electrical potential in the x-z plane at y=Y1 in FIG. 17A; while a line “b′” in FIG. 19 represents the changes of the electrical potential in the x-z plane at y=Y1 in FIG. 17B. Referring to FIG. 19, it is found that the line “b′” has more moderate and wider inclination of the changes of the electrical potential in the x-direction than the line “b”.
That is, when Vc′=(½)×Vc, the inclination of the changes of the electrical potential decrease (become smaller) in a boundary area between the image area and the non-image area in the y-direction and the x-direction. Therefore, the developing position (adhering position) of the toner becomes unstable near the boundary area as shown in FIG. 20B. It is understood that the unstableness is the cause of the “coarseness”.
Therefore, when reducing the toner bearing amount, in order to prevent the coarseness from worsening, it is preferable to perform the image forming at a maximum developing contrast Vc equal to or greater than the conventional level.
(3) Worsening of Fogged Image
As for the fogged image; i.e., about a phenomenon of toner adhesion to the non-image area during developing process, the following fact was found. That is, since the toner bearing amount is reduced and the tinting strength of the toner is increased at the same time, the frequency of fogged images tends to be the same as or worse than the conventional art.
As described above, in order to reduce the toner bearing amount, just simply reducing the developing contrast to reduce the toner bearing amount by increasing the tinting strength of the toner and utilizing the thus increased density may decrease the stability and image quality. That is, such problems as unstableness, worsening of coarseness and fogged images may occur. As described above, it is requested to increase the productivity, to reduce the power consumption, the toner relief and the running cost while enabling the reduction of the toner bearing amount without decreasing the conventional stability and the image quality.